The present invention relates to a method and an apparatus for continuous casting of metals, comprising a casting mould with an elongated horizontal cross section, through which a molten metal is intended to pass during the casting operation, a member for supplying a molten metal to such molten metal already present in the casting mould in a region at a distance below the upper surface of the latter melt, and a device adapted to apply magnetic fields to the melt in the casting mould to influence movements of the molten material.
An apparatus of the above-mentioned type is illustrated schematically in the accompanying FIG. 1. From a so-called tundish 1, a molten metal 2 is supplied to a casting mould 3 in the form of a box, open at the top and at the bottom, having cooled walls, usually of a copper-based alloy with a good thermal conductivity. The cooling in the casting mould causes the solidification of the elongated strand, formed by the molten metal, to begin from the outside and proceed inwards towards the centre of the strand. During casting with the above-mentioned cross section of the casting mould, a strand is formed which is usually referred to as a slab. The cooled and partially solidified strand continuously leaves the casting mould. At a point where the strand leaves the casting mould, it has at least one mechanically self-supporting, solidified casing 4 that surrounds a non-solidified centre 5. It is shown schematically how it is sufficient with guide rollers S to guide and support the strand downstream of the casting mould.
For the further explanation of the field of the invention, a brief reference is also made to part of FIGS. 2a and 2b, although the apparatus shown therein does not belong to the prior art but to the present invention. From the tundish 1 extends a casting pipe 6 for supplying the hot molten metal into the molten metal already present in the casting mould 3 at a distance, preferably a considerable distance, below the upper surface 7 of the latter melt, this surface being usually referred to as the meniscus. The melt flows out of the casting pipe 6 in laterally located openings therein and thereby generates a so-called primary flow as well as a so-called secondary flow. These flows are schematically indicated by the dashed arrows in FIG. 2b. The primary flow 8 extends downwards in the casting direction, whereas the secondary flow 9 extends from the area of the walls 10 of the casting mould upwards towards the upper surface of the molten bath and then downwards. In different parts of the molten bath that exists in the casting mould, or the mould, periodic velocity fluctuations arise in the cast material during the casting process. These fluctuations are also due to the walls of the casting mould being normally set into an oscillating movement to prevent solidified cast material from adhering thereto. The irregular movements caused thereby in the molten metal implies, inter alia, that bubbles, for example argon gas bubbles, and impurities in the melt, for example oxide inclusions from the casting pipe and slags from the meniscus, are transported far down in the casting direction, that is, far down in the cast strand that is initially formed in the casting mould. This results in inclusions and irregularities of the finished, solidified cast strand. These problems become especially great in the case of high casting speeds, that is, when a large volume of molten material is supplied to the casting mould per unit of time.
This also entails a considerable risk of irregular speeds of the movements of the molten material in the area of the upper surface of the bath and of resultant pressure variations at the upper surface, and a risk that variations in height may occur in the upper surface. At high casting speeds, this leads to slag being drawn down, uneven slag thickness, uneven shell thickness, and a risk of formation of cracks. There is also a risk of oscillations of the molten material in the casting mould leading to an unsymmetrical speed of the cast material downwards in the mould, such that the speed at one side becomes considerably higher than the speed at the other side. This results in a considerable transport downwards of inclusions and gas bubbles with an ensuing deteriorated slabs quality.
Thus, for the casting result, it is important to achieve a speed of the molten metal downwards in the casting mould that is essentially uniform over the cross section of the casting mould, that is for the primary flow, and a stable upwardly-directed flow at the short sides of the casting mould so that the movements of the molten metal in the area of the upper surface of the molten bath become constant in time and such that a uniform, stable temperature is achieved at the upper surface of the melt.
It is for this reason that a device as mentioned above (indicated at 11 in FIG. 1) is arranged to apply magnetic fields to the melt in the casting mould. In this context, a plurality of various ways of influencing the movement of the molten material by applying magnetic fields have been suggested. One way is to utilize the so-called EMBR (ElectroMagnetic BRake) technique, in which a stationary magnetic field, that is, a magnetic field generated by leading a direct current through a coil of an electromagnet, is applied to the melt in the casting mould from one long side to the other. This then results in the movements of the molten material being braked. In this context, such electromagnets may be arranged along the casting mould in the vicinity of, or below, the region for the supply of molten metal in order thus to brake the flow of the molten metal downwards in the casting mould, that is, substantially to influence the primary flow mentioned, to try to render the speed of this movement essentially constant over the whole cross section of the casting mould, and to stabilize the upwardly-directed secondary flow at the short sides of the casting mould. However, it is also possible to arrange a so-called brake in the area of the upper surface of the casting mould to brake the movements of the molten metal in this area and remove surface oscillations in the melt. These two locations of electromagnetic brakes may also be combined into a so-called FC (Flow Control) mould, which is previously known from, for example, JP 97357679.
Another way of influencing movements of the molten material in the casting mould by applying a magnetic field to the melt in the casting mould is previously known from, for example, U.S. Pat. No. 5,197,535 and is referred to as EMS (=Electromagnetic Stirring). Here, by connecting a polyphase ac voltage to electromagnets along the casting mould, a travelling magnetic field is generated, which is usually applied in the area of said upper surface to guide the movements of the molten material in this area. This is, therefore, of interest especially at lower casting speeds, since there is then a risk that the movement of the cast material in the area of the upper surface will be too small and that temperature differences, which have a negative influence on the casting result, may arise.
Also other apparatuses for influencing movements of the molten material, by applying magnetic fields to the melt in a casting mould for continuous casting, are previously known.